The term “silicon” encompasses all Si grade stages, especially metallurgical silicon which has been obtained directly from the carbothermal reduction of SiO2, UMG silicon (UMG=upgraded metallurgical grade), solar-grade silicon, and electronic-grade silicon, and also the associated raw silicon and the corresponding precursors of the Si grade stages and of the corresponding raw silicon.
Metallurgical silicon encompasses all Si grade stages which do not meet the purity criteria for semiconductor applications. Metallurgical silicon or UMG silicon are not suitable for production of solar cells or applications in the electronics industry since they still contain relatively high concentrations of impurities which for these purposes are unwanted and must be removed.
For the removal of impurities from silicon melts it is possible to employ gaseous halogen sources. More particularly, halogen gas, halogen-containing gas mixtures or gaseous, halogen-containing compounds are introduced into the Si melt. For example, DE 3635064 C2 discloses a method for purifying silicon by treating a silicon melt with a gas mixture comprising hydrogen chloride and/or halosilanes, and by subsequent vacuum treatment at less than 0.1 mbar.
Implementing a technology of that kind, however, is very complex, since the halogen or the gaseous, halogen-containing compounds must be introduced directly into the melt which is generally accomplished by tubes or special nozzles. Consequently, the possibility for homogeneous distribution of the halogen over the entire melt is limited. Moreover, the devices for introducing the halogen into the melt may impair the melt itself. This means that, for example, there may be impurities which originate from the gas introduction devices.
WO 2009/143825 A2 describes a method for purifying metallurgical silicon by admixing metallurgical silicon with solid, halide-containing silicon, preparing a melt from the substances, and sublimating out the impurities from the melt and removing them in the form of metal halides.
Implementing a technology of that kind, however, is dependent on preparation of the halide-containing silicon needed for the purpose, that preparation generally taking place from halogenated polysilanes. In that case, those halogenated polysilanes must first be converted into a material whose halogen content is reduced significantly in comparison to the starting material (and also into volatile halosilanes having an increased halogen content in comparison to the starting material). The material obtained in that conversion is the halide-containing silicon.
It could thus be helpful to provide an improved method for purifying silicon that is simplified.